There's no good reason not to. Pretty birds taste just the same as ugly birds when cooked properly. And while they may be rare, that alone is no reason to keep them. In fact, IMO most breeds would benefit greatly if more people would eat the "pretty" birds that just don't fit the standard (or at least get them closer to fitting the standard) instead of breeding them just because they are a "rare" color. In fact, we are looking at getting some orpingtons to try as meat birds soon and I am specifically avoiding the lavender color because they are a project bird and still have a bit of work to be done with them before they are at the same level conformation wise as the more common, already established colors. I don't really have the time to put into a project like that right now. Color alone is not what makes the bird, it's just the icing on the cake. I'd let them grow out a little more, especially since orpingtons are known for being a little slower to mature and can often be late bloomers, and then keep the one that is closest to the breed standard and eat the other one.
As for the legs, the shank color may be dark but the rest of the skin should be white. Last weekend we processed a bunch of blue and black ameraucana mixes. More than half of them had dark shanks, but all of them had white skin once we plucked them. And speaking from personal experience, dark shanks make just as good a broth as light shanks.
